Unique chemical mechanical planarization approach which utilizes magnetic slurry for polish and magnetic fields for process control

ABSTRACT

The present invention is an improved apparatus and process for chemical mechanical polishing (CMP) layers which have a low dielectric constant (k). The present invention uses a magnetic slurry and a magnetic coil for polishing the wafer with the magnetic slurry. By using a magnetic slurry and a magnetic coil the force used during polishing can be controlled resulting greater control over the CMP process during the polishing of low k materials.

This is a division of application Ser. No. 09/001,509, filed Dec. 31,1997, now U.S. Pat. No. 6,083,839.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to the field of semiconductor devices, and morespecifically, to a process and apparatus for chemical mechanicalpolishing.

2. Background Information

Integrated circuits manufactured today are made up of literally millionsof active devices such as transistors and capacitors formed in asemiconductor substrate. These active devices are formed andinterconnected in an elaborate system of layers. A considerable amountof effort in the manufacture of modern complex, high density multilevelinterconnections is devoted to the planarization of the individuallayers of the interconnect structure. Nonplanar surfaces create pooroptical resolution of subsequent photolithographic processing steps.Poor optical resolution prohibits the printing of high densityinterconnect metal lines. Another problem with nonplanar surfacetopography is the step coverage of subsequent metallization layers. If astep height is too large there is a serious danger that open circuitswill be created. Planar interconnect surface layers are a must in thefabrication of modern high density integrated circuits.

To ensure planar topography, various planarization techniques have beendeveloped. One approach, known as chemical mechanical polishing, employspolishing to remove protruding steps formed along the upper surface ofinterlayer dielectrics (ILDs). Chemical mechanical polishing is alsoused to “etch back” conformally deposited metal layers to form planarplugs or vias.

FIG. 1 illustrates a typical chemical mechanical polisher 100. As shown,a substrate (or wafer) 110 is held by a carrier 120. Carrier 120 presseswafer 110 against polishing pad 130 with a downward force. Polishing pad130 is attached to polishing platen 140. Polishing pad 130 is coveredwith an active slurry 150 and polishing platen 140 rotates in onedirection while carrier 120 rotates in the opposite direction. Thedownward force, rotational motion, surface of the polishing pad, andslurry act together to polish or planarize the surface of wafer 110.

This type of chemical mechanical polishing however exhibits someproblems. For example, such tools have limitations in controlling bothcomponents of shear force and downward force applied to a wafer surface.Moreover, as semiconductor devices become smaller and more densechemical mechanical polishing is causing some problems with newermaterials used to fabricate current semiconductor devices. Prior artmaterials used in conjunction with chemical mechanical polishing havebeen relatively hard and/or stiff materials such as oxides, polysilicon,etc. As a result, chemical mechanical polishing processes have beenoptimized for these materials.

New materials, such as materials with low dielectric constants are beingused in order to reduce the RC Time Constant in current semiconductordevices. The RC Time Constant is the fundamental limit of amicroprocessor caused by the capacitance between the metal lines of themicroprocessor. There are two things which determine the RC TimeConstant: the resistance of the metal lines themselves and thecapacitance of the dielectric materials.

Silicon dioxide, which is widely used as a dielectric material has adielectric constant (k) of approximately k=4. However, by switching tomaterials with lower dielectric constants, for example in the range ofapproximately k=2-3, several advantages may be obtained. The use of lowk polymers have been found reduce the RC Time Constant due to adecreased capacitance and therefore increase the speed of the device.The use of low k materials have also been found to reduce powerconsumption, and reduce crosstalk noise between metal lines.

Unfortunately, low k materials tend to be polymers which are moreplastic like materials. Therefore, when polishing such low k materialsin chemical mechanical polishing they tend to bend and/or deform,because they are plastic, causing bad results and bad uniformity duringplanarization. This is especially apparent when comparing the polishingresults in areas with densely populated regions of the topography andlightly populated regions of the topography. In other words, where thetopography is densely populated the low k materials are less likely tobend under the downward force of the polisher resulting in betterpolishing uniformity. Whereas, in areas of the topography that are morelightly populated the low k materials are more likely to deform underthe downward force of the polisher causing nonuniform results in thepolishing process.

FIG. 2 illustrates a low k material after planarization with prior artchemical mechanical polisher and polishing method. As shown, low kmaterial 210 was deposited above metal lines 220 and substrate 200.Since low k material 210 is somewhat plastic it deformed during thechemical mechanical polishing process. As illustrated, because low kmaterial 210 deformed more in the less populated areas of the topographyduring the polishing process and as a consequence the top surface is notuniform and is not evenly planarized.

Other process issues also arise during chemical mechanical polishing oflow k materials. For example, low k materials may interact with thepolishing pads and slurries of the chemical mechanical polishingprocess. As another example, low k materials are mechanically weak andmay have poor adhesion (compared to silicon oxides) and therefore maynot hold up or adhere to underlying layers through the chemicalmechanical polishing process. Additionally, low k materials have a lowerthermal stability which may be affected by the friction and highertemperatures of the chemical mechanical polishing process. At highertemperatures low k materials may suffer thermal deformation (andplasticity) due to material heat up.

Thus, what is needed is a chemical mechanical polisher and polishingprocess that will improve process control over the planarization of lowk materials with polishing uniformity and good electrical resultsregardless of the density of the underlying topography.

SUMMARY OF THE INVENTION

The present invention is an improved method and apparatus for polishing.The apparatus includes a wafer holder, a magnetic slurry, and a magneticcoil.

Additional features and benefits of the present invention will becomeapparent from the detailed description, figures, and claims set forthbelow.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is illustrated by way of example and notlimitation in the accompanying figures in which:

FIG. 1 illustrates a typical chemical mechanical polisher used in theprior art.

FIG. 2 illustrates a low k material after planarization with prior artchemical mechanical polisher and polishing method.

FIG. 3 illustrates a chemical mechanical polisher of the presentinvention.

FIG. 4 illustrates a low k material after planarization with thechemical mechanical polishing apparatus and process of the presentinvention.

DETAILED DESCRIPTION

A Unique Chemical Mechanical Planarization Approach which UtilizesMagnetic Slurry for Polish and Magnetic Fields for Process Control isdisclosed. In the following description, numerous specific details areset forth such as specific materials, process parameters, equipment,etc. in order to provide a thorough understanding of the presentinvention. It will be obvious, however, to one skilled in the art thatthese specific details need not be employed to practice the presentinvention. In other instances, well known materials or methods have notbeen described in detail in order to avoid unnecessarily obscuring thepresent invention.

The present invention is an improved apparatus and process for chemicalmechanical polishing (CMP) layers which have a low dielectric constant(k). The present invention uses a magnetic slurry and a magnetic coilfor polishing the wafer with the magnetic slurry. By using a magneticslurry and a magnetic coil the force used during polishing can becontrolled resulting in greater control over the CMP process during thepolishing of low k materials.

It should be noted that, the process steps and structures describedbelow do not form a complete process flow for manufacturing integratedcircuits. The present invention can be practiced in conjunction withintegrated circuit fabrication techniques currently used in the art, andonly so much of the commonly practiced process steps are included as arenecessary for an understanding of the present invention. The figuresrepresenting portions of an integrated circuit during fabrication arenot drawn to scale, but instead are drawn so as to illustrate theimportant features of the invention.

It should also be noted that, the terms wafer and substrate are beingused herein interchangeably. Also, that reference to a wafer orsubstrate may include a bare or pure semiconductor substrate, with orwithout doping, a semiconductor substrate with epitaxial layers, asemiconductor substrate incorporating one or more device layers at anystage of processing, other types of substrates incorporating one or moresemiconductor layers such as substrates having semiconductor oninsulator (SOI) devices, or substrates for processing other apparati anddevices such as flat panel displays, multichip modules, etc.

As stated in the background of the invention, low k dielectricmaterials, such as polymers, are being used in order to reduce the RCtime constant, reduce power consumption, and reduce metal line crosstalk in order to improve the performance of semiconductor devices. Suchlow k materials however deform/bend, interact with polishing pads andslurries, and exhibit poor adhesive qualities in prior art chemicalmechanical polishing (CMP) processes and apparati. Some examples of lowk materials are polyaromatic ethers (PAEs), aerogels, xerogels,parylene, and amorphous fluorocarbons.

In order to improve planarization uniformity and process control of lowk materials, the present invention changes the forces used during theCMP process. By changing the forces from mechanically applied downwardcompressive force to a magnetically controlled force and slurry, thepresent invention improves the distribution of the slurry and polishingresults regardless of the density of the population of the topography.

FIG. 3 illustrates a chemical mechanical polisher 300 as used in anembodiment of the present invention. As shown, a substrate (or wafer)310 is held by a wafer holder 320. In one embodiment of the presentinvention wafer holder 320 is an electrostatic chuck. It should be notedthat the wafer 310 may be recessed in the wafer holder 320 for optimaldelivery of the slurry however, other embodiments may hold the wafer ontop of wafer holder 320.

As illustrated, above the wafer is a magnetic slurry 330. The magneticslurry 330 may be a silica or cerium based slurry, for example silicaoxide and cerium oxide, having a ferromagnetic material therein. Itshould be noted that the chemical portion of the slurry is the silica orcerium base while the portion of the magnetic slurry that imparts thephysical motion is the ferromagnetic material. It should also be notedthat the magnetic slurry 330 may be a wet or dry polishing slurry.

Chemical mechanical polisher 300, as shown, also has a movable stage 350which includes magnetic coils 340. Movable stage 350 and magnetic coils340 are used to impart motion to magnetic slurry 330 for the polishingprocess. The strength of magnetic coils 340 are to be sized such thatthe optimal polishing parameters may be obtained. For example strongermagnetic coils increase the force of the magnetic slurry on the wafersurface. Stronger magnetic coils may also be used to increase thepolishing rate. The field strength of magnetic coils 340 may range fromapproximately 100-1,000 Gauss. In one embodiment of the presentinvention the magnetic coils 340 have a field strength in the range ofapproximately 800-900 Gauss.

It should be noted that the field strength of the magnetic coils mayvary within the movable stage 350. For example, the first magnetic coilmay have a field strength of approximately 500 Gauss, while the secondmagnetic coil may have a field strength of approximately 800 Gauss, thethird magnetic coil may have a field strength of approximately 700Gauss, etc. It should also be noted that there may be any number ofmagnetic coils in the movable stage and the number of magnetic coilswill depend upon the particular user.

Movable stage 350 imparts the motion to the magnetic slurry 330. Movablestage 350 may be moved in an any desired direction in order to move theslurry to desired locations or perform a particular polish pattern.Movable stage may be moved in an x-y directional plane (360) as well asin a circular motion (not shown). Movable stage may also be moved up anddown, or closer to and further from (not shown) the wafer holder 320 inorder to vary the strength of the magnetic coils on the magnetic slurry.In other words, the closer the movable stage 350 is to the wafer holder320, the greater the strength of the magnetic coils 340 and the greaterthe force exhibited on the magnetic slurry 330. Also, the further themovable stage 350 is from the wafer holder 320, the lesser the strengthof the magnetic coils 340 and the lesser the force exhibited on themagnetic slurry 330.

Imparting motion to magnetic slurry using movable stage 350 andcontrolling the downward force of the magnetic slurry by changing thefield strength and positioning of the magnetic coils allows greaterprocess control over the CMP process especially in the polishing and/orplanarization of low k materials. It should be noted that fieldmodelling would be advantageous in order to determine the bestconditions for field uniformity and strength of the magnetic coils.

FIG. 4 illustrates a low k material after planarization with thechemical mechanical polishing apparatus and process of the presentinvention. As shown, low k material 410 was deposited above metal lines420 and substrate 400. Since, with the use of the present invention, thedownward force and slurry distribution has greater control, the polishedlow k material 410 is more uniform and does not exhibit as muchdeformity during CMP. Therefore, as illustrated, with the use of thepresent invention the top surface of the low k material 410 is uniformand is more evenly planarized regardless of the underlying topography.

It should be noted that since the present invention does not use apolishing pad there is no concern with the interaction between the low kmaterial and a polishing pad material. It should also be noted thatbecause of the greater control of the downward and shear force with thepresent invention the low k material will not be ripped or tugged at andtherefore the weakness and poor adhesive qualities of the low k materialare also not as great a concern.

Additionally, the only friction in the present invention is between theslurry and the wafer being polished. Therefore, there is no longer thefriction component from the polishing pad against the wafer as in theprior art. As stated in the background section the friction of thepolishing pad and wafer tended to heat up the CMP process and causedproblems because of the low k materials have a lower thermal stabilityor deformation temperature. Because such friction no longer exists inthe present invention the temperature of the CMP process will remainrelatively steady and the lower thermal stability of the low k materialsis not as great a concern.

Thus, a Unique Chemical Mechanical Planarization Approach which UtilizesMagnetic Slurry for Polish and Magnetic Fields for Process Control hasbeen described. Although specific embodiments, including specificequipment, parameters, methods, and materials have been described,various modifications to the disclosed embodiments will be apparent toone of ordinary skill in the art upon reading this disclosure.Therefore, it is to be understood that such embodiments are merelyillustrative of and not restrictive on the broad invention and that thisinvention is not limited to the specific embodiments shown anddescribed.

What is claimed is:
 1. An apparatus comprising: a substrate holder; asubstrate disposed on top of said substrate holder; a magnetic slurrydisposed on top of said substrate; and a stage disposed below saidsubstrate holder, said stage comprising a magnetic coil.
 2. Theapparatus of claim 1 wherein said substrate is a wafer.
 3. The apparatusof claim 2 wherein said substrate holder is a wafer holder.
 4. Theapparatus of claim 3 wherein said wafer holder is an electrostaticchuck.
 5. The apparatus of claim 3 wherein said wafer is recessed insaid wafer holder.
 6. The apparatus of claim 1 wherein said magneticslurry comprises a ferromagnetic material.
 7. The apparatus of claim 6wherein said magnetic slurry further comprises siilica oxide.
 8. Theapparatus of claim 6 wherein said magnetic slurry further comprisescerium oxide.
 9. The apparatus of claim 1 wherein said magnetic slurrycomprises a wet polishing slurry.
 10. The apparatus of claim 1 whereinsaid magnetic slurry comprises a dry polishing slurry.
 11. The apparatusof claim 1 wherein said strength can increase a force of said magneticslurry on said substrate.
 12. The apparatus of claim 1 wherein saidstrength can increase a polishing rate of said substrate.
 13. Theapparatus of claim 1 wherein said magnetic coil has a field strengththat may range from approximately 100-1000 Gauss.
 14. The apparatus ofclaim 1 wherein said magnetic coil has a field strength in a range ofapproximately 800-900 Gauss.
 15. The apparatus of claim 1 wherein saidmagnetic coil may have a field strength that may vary within said stage.16. The apparatus of claim 1 wherein said stage may be moved in anydesired direction in order to move said magnetic slurry to desiredlocations.
 17. The apparatus of claim 1 wherein said stage may be movedin any desired direction in order to perform a particular polishpattern.
 18. The apparatus of claim 1 wherein said stage may be moved inan x-y directional plane as well as in a circular motion.
 19. Theapparatus of claim 1 wherein said stage may be moved closer to orfurther from said substrate holder in order to vary said strength ofsaid magnetic coil on said magnetic slurry.
 20. The apparatus of claim 1wherein said apparatus does not comprise a polishing pad.
 21. Anapparatus comprising: a wafer holder, said wafer holder having a firstside and a second side; a magnetic slurry disposed adjacent to saidfirst side of said wafer holder; and a stage disposed adjacent to saidsecond side of said wafer holder, said stage comprising a magnetic coil.22. The apparatus of claim 21 wherein said magnetic coil has a strengththat is sized to obtain optimal polishing parameters.
 23. The apparatusof claim 21 wherein said stage may be moved in any desired direction inorder to perform a particular polish pattern.
 24. The apparatus of claim22 wherein said stage may be moved closer to or further from said waferholder in order to vary said strength of said magnetic coil on saidmagnetic slurry.